1. Field of the Invention
The present invention relates to aqueous ink compositions for use in writing instruments such as of direct ink supply type or of wick type. More particularly, the present invention relates to aqueous ink compositions capable of exhibiting their intrinsic deep colors such as navy blue, violet, green and brown even on black paper for use in writing instruments of direct ink supply type having an ink container tube directly connected therewith and/or provided with an ink flow adjusting mechanism, through which ink container tube the ink remaining therein is visible, and further to aqueous ink compositions free from clogging at a writing tip even after prolonged use and excellent in concealing power for use in writing instruments of wick type.
2. Related Arts
Writing instruments of so-called direct ink supply type have a transparent ink container adapted to be directly charged with ink for storage and a ball-point tip, felt tip or the like writing tip. Writing instruments of wick type have a wick formed of a bundle of fibers with its periphery covered with a resin or the like and impregnated with ink, from which the ink is supplied to a ball-point tip, felt tip or the like writing tip through capillarity.
It has been a conventional practice to prepare an aqueous ink composition for use in such writing instruments by adding a water-soluble organic solvent of a polyvalent alcohol or a derivative thereof to an aqueous solution containing a water-soluble dye or a water-dispersible pigment.
However, a conventional aqueous ink composition prepared by adding thereto a certain pigment for impartation of a deep color fails to exhibit its intrinsic color on black paper because the color of the ink is assimilated with the black of the paper. An ink composition of a deep color applied to a transparent ink container tube of a refill for the writing instruments of direct ink supply type exhibits a deeper color in the container tube than that it exhibits on paper. Specifically, an ink composition of navy blue, violet, green, brown or the like looks almost black in the container tube, making it impossible to visually recognize the color thereof through the transparent ink container tube. Attempts have been made to overcome the aforesaid drawbacks by adding titanium oxide to an ink composition. However, the ink composition containing titanium oxide suffers from clogging at the writing tip of a writing instrument, because titanium oxide has a high specific gravity and cannot be rendered into fine particles. Thus, a satisfactory solution to the problem is yet to be found.
For application to the writing instruments of wick type, there has been proposed an aqueous ink composition to which titanium oxide is added to impart thereto concealing power. If a writing instrument charged with such ink composition is allowed to stand with its writing tip oriented downward for a long time, however, titanium oxide having a high specific gravity precipitates into the writing tip to cause clogging and thus the instrument is disabled for use. To solve this problem, Japanese Unexamined Patent Publications No. 63-145380 (1988), No. 63-145382 (1988) and No. 2-133479 (1990) disclose aqueous pigment ink compositions containing titanium oxide along with a hollow resin emulsion called "plastic pigment", all of which have a reduced concealing power compared with that of an ink composition not containing the hollow resin emulsion.
It is, therefore, an object of the present invention to provide an aqueous ink composition which assures long-tern stability in dispersibility while maintaining an excellent concealing power, and is free from clogging at a writing tip even after a long-term storage, and the color of which can be recognized through a transparent ink container.